1. Field of the Invention
This invention relates to a method of producing, by employing electron beam irradiation, a low iron loss grain oriented silicon steel sheet which generates improved magnetostrictive characteristics when used as a stacked iron core and low noise when used in a stacked transformer, as well as superior shape characteristics.
Grain oriented silicon steel sheets are used mainly as the core materials of electrical components such as transformers or the like. In general, grain oriented silicon steel sheets are required to have such magnetic characteristics that the magnetic flux density (represented by B.sub.8) is high and that the iron loss (represented by W.sub.17/50) is low. It is also required that the surfaces of the steel sheet have insulating films with excellent surfaces.
The energy crisis that the world now faces requires reduction of losses of electrical power. This has given rise to a strong demand for grain oriented silicon steel sheets having reduced iron loss.
2. Description of the Related Art
Grain oriented silicon steel sheets have undergone various treatments for improving magnetic characteristics. For instance, treatment has been conducted to attain a high degree of concentration of the secondary recrystallization grains in the Goss orientation. It has also been attempted to form, on a forsterite film formed on the surface of the steel sheet, an insulating film having a small thermal expansion coefficient so as to impart a tensile force to the steel sheet. Thus, grain oriented silicon steel sheets have been produced through complicated and diversified processes which require very strict controls.
Among these treatments, one major technique for reducing iron loss of grain oriented steel sheet has been the improvement of the aggregation structure of Goss orientation secondary recrystallization grains.
Hitherto, as a method for controlling the secondary recrystallization grains, preferential growth of Goss orientation secondary recrystallization grains has been obtained by using primary recrystallization grain growth inhibiting agents such as AlN, MnS and MnSe, known as "inhibitors".
In recent years various techniques other than metallurgical measures have been developed for controlling secondary recrystallization grains for reducing iron loss. For instance, techniques for reducing iron loss by irradiation with laser beams have been proposed in IRON AND STEELS, by Tadashi Ichiyama 69(1983), P895, Japanese Patent Publication No. 57-2252, Japanese Patent Publication No. 57-53419, Japanese Patent Publication No. 58-26405 and Japanese Patent Publication No. 58-26406. Methods also have been proposed which employ plasma irradiation as disclosed, for example, in Japanese Patent Laid-Open No. 62-96617, Japanese Patent Laid-Open 62-151511, Japanese Patent Laid-Open No. 62-151516 and Japanese Patent Laid-Open No. 62-151517. In these methods local treatment is introduced into the steel sheet by irradiation of the steel sheet surface by laser beam or plasma, so as to refine the magnetic domains, thereby reducing iron loss.
These methods relying upon irradiation with laser beam or plasma, however, inevitably raise the cost of reducing iron loss, because the energy efficiency is as low as 5 to 20%.
Under these circumstances we have proposed a method in which an electron beam generated by electric power of high voltage and low current is locally and intermittently applied along the widthwise direction which intersects the rolling direction of the sheet, so as to forcibly introduce a coating film into the matrix iron. Such a method is disclosed, for example, in Japanese Patent Laid-Open No. 63-186826, Japanese Patent Laid-Open No. 2-118022 and Japanese Patent Laid-Open No. 2-277780.
This method exhibits very high energy efficiency, as well as high scanning speed, thus offering remarkably improved production efficiency as compared to known methods for refining magnetic domains.
The methods disclosed in our above-mentioned Japanese Patent Laid-Open specifications are directed to production of grain oriented silicon steel sheet for use as a material for a wound core transformer. In the production of a core of this kind, the wound core formed from a grain oriented steel sheet is subjected to stress-relieving annealing. Therefore, no substantial noise tends to be generated in the wound core transformer during operation of the transformer.
In contrast, a stacked transformer of that kind generates a high level of noise, requiring strong measures to be taken for reducing the noise.
In particular, the grain oriented steel sheets produced by the method proposed in the aforementioned Japanese Patent Laid-Open specification cannot be practically used in stacked transformers, due to high levels of noise.
Furthermore, these techniques are still unsatisfactory in that the products exhibit large fluctuations in magnetostrictive and sheet shapes, making it difficult to stably produce steel sheets having acceptable product quality.
On the other hand, U.S. Pat. No. 4,919,733 discloses a method for refining magnetic domains by irradiation with electron beams, wherein the surface energy density on the electron beam scan line is set to a level not lower than 60 J/in.sup.2 (9.3 J/cm.sup.2).
Steel sheets which have undergone this electron beam treatment, however, exhibit inferior noise characteristics when employed in a stacked transformer, as compared with steel sheets which have not undergone such electron beam treatment. In particular, the noise characteristics are extremely poor during operation of the transformer after the electron beam treatment has been conducted under the conditions mentioned above, as compared with sheets which have not undergone such treatment.
U.S. Pat. No. 4,915,750 proposes a method of producing a grain oriented silicon steel sheet for use as a material of a wound core transformer, employing refining of magnetic domains by irradiation with an electron beam. This method is directed only to the production of a wound core transformer as distinguished from a stacked transformer to which the present invention pertains and which suffers from the noise problem.